In the conventional basic oxygen method for converting molten pig iron to steel a quantity of molten metal is contained in a refractory lined vessel and oxygen is injected directly into the surface of the molten metal by means of a lance which is admitted through the mouth of the vessel while it is upright. The vessel is usually supported in a trunnion ring from which diametrically opposite trunnion pins extend into bearings which facilitate tilting the vessel for the purposes of charging it with raw materials and for discharging its contents through its mouth. There is usually a hood above the mouth of the vessel when it is in its upright position for the purpose of collecting gases which evolve incidental to the steel refining process. When it is desired to charge the vessel with hot metal, scrap and slag producing and fluxing agents such as burnt lime, the vessel mouth must be tilted away from the hood or the hood must be shifted to provide access to the vessel. Large quantities of smoke and noxious gases evolve from the vessel into the atmosphere when the vessel and gas collection hood are separated. Even when the hood is close to the vessel mouth as it is during vessel operation, significant quantities of smoke and gases still escape to the atmosphere. This is a major source of pollution which has resulted in threats to shut plants down until antipollution regulations can be met.
Bottom blown oxygen converter vessels have become known recently. In this type of vessel oxygen and finely divided fluxing materials are injected primarily beneath the surface of the molten metal within the vessel through tuyeres in the bottom and sides of the vessel. By injecting oxygen, other gases and finely divided solids entrained in gases through the bottom tuyeres, the constituents of the molten metal, such as the carbon which is to be reduced by oxygen, are in intimate contact with the gases and solids that permeate the melt. This results in more nearly stoichiometric chemical reactions and there is an accompanying reduction of smoke. Moreover, it is possible to keep the hood very close to the vessel mouth so that only insignificant quantities of gas and smoke can escape to the atmosphere.